You know the date, but do you know what day of the week it fell on? Whether you are digging up an old receipt, planning a surprise birthday party, booking a flight that leaves on a Tuesday, or double-checking what weekday your wedding lands on, the calendar is easy to misread in your head. This day of week calculator takes any year, month, and day and instantly tells you the weekday — Monday through Sunday — plus whether it is a weekend day and which day of the year it is.
The math underneath is the same rule your phone uses: the proleptic Gregorian calendar assigns every date an ordinal number, and that number modulo 7 cycles through the seven weekdays. You do not have to trust the arithmetic blindly; the engine counts leap years correctly (century years divisible by 400 are leap, other century years are not), so February 29 and New Year's Day both resolve to the right weekday.
How to Use
- Enter the Year as a four-digit number, such as 2026. The supported range is 1900 through 2200, which covers essentially every date anyone plans around in practice.
- Enter the Month as a number from 1 (January) to 12 (December). If you only remember a date as "October 24," type 10 for the month.
- Enter the Day of the month, from 1 to 31. The tool automatically shortens February in common years and lengthens it in leap years, so you never have to remember which months have 30 days.
- Read the result card: the bold Day of the Week is your answer, followed by the weekday index (Monday = 0, Sunday = 6), a weekend flag, and the day-of-year count.
- Adjust any of the three boxes and the result updates immediately. Type a few candidate dates back to back to compare weekdays for meetings, deadlines, or travel.
If the date you enter is impossible — for example, month 0, day 31 in February, or a year outside 1900-2200 — the calculator returns a clear error instead of a fabricated weekday. That keeps bad inputs from quietly producing nonsense.
Why the Weekday Keeps Shifting
The calendar repeats every 400 years, but never from one year to the next. A common year has 365 days, which is 52 weeks plus 1 day, so every fixed calendar date slips forward by one weekday each year. In a leap year the slip is two days after February. That is why your friend's birthday on March 12 landed on a Wednesday last year and on a Thursday this year, and why "the first Monday of the month" is a moving target you cannot just memorize.
Day-of-year counting also matters. On October 24, 2026, the year is already 297 days old, which is why so many annual deadlines (tax extensions, warranty expirations, annual leave caps) land in predictable administrative windows.
Examples
Example 1 — A known Saturday. Enter year 2026, month 10, day 24. The calculator returns Saturday, weekday index 5, weekend flag Yes, and day of year 297. This is the anchor date used to verify the engine: compare it against a printed calendar and it matches.
Example 2 — New Year's Day. Enter 2026, month 1, day 1. The result is Thursday, index 3, day of year 1. If your gym offers a 15% New Year's discount that "runs the first Saturday in January," you now know January 1, 2026 is a Thursday — so the discount actually kicks off on January 3 instead.
Example 3 — Planning around rent. Suppose your rent of $1,850 is due on the 1st of every month and your bank only processes transfers on weekdays. Enter the upcoming 1st for March, June, and September of the same year. Whichever 1st lands on a Saturday or Sunday should be paid by the prior Friday, because landlords rarely forgive a weekend-timing mistake, and a single late fee of $50 to $75 is painful on top of the $1,850 itself.
Example 4 — A leap-year date. Enter 2024, month 2, day 29. The calculator correctly accepts the date (2024 is a leap year) and returns Thursday, day of year 60. Try February 29, 2023 and the engine rejects it as invalid, because 2023 is not a leap year.
Example 5 — Going far back or far ahead. Enter 1900, month 1, day 1 and you get a Monday; enter 2200, month 12, day 31 and the engine still resolves a valid weekday. Anything outside that window is refused with an error rather than extrapolated.
FAQ
What day of the week was I born?
Enter your birth year, birth month, and birth day into the three boxes above. The result card shows the exact weekday of your birthday. Because leap years and the 365-day slip shift weekdays every year, this is almost never the weekday you currently celebrate on — it is the true weekday you were born on.
How does the calculator know the weekday without a calendar?
Every Gregorian date can be converted to an ordinal number of days since a fixed epoch. Taking that number modulo 7 gives a value from 0 to 6, which maps directly to Monday through Sunday. Leap years are handled by the rule that a year is leap if it is divisible by 4, except century years must also be divisible by 400. This is the same arithmetic used by standard date libraries.
Why do weekdays shift every year?
A common year is 365 days = 52 weeks + 1 day, so each fixed calendar date moves forward one weekday. A leap year adds a second shift for dates after February. Over a full 400-year cycle (146,097 days, exactly divisible by 7) the whole calendar repeats, which is why the supported range caps cleanly and reliably at 1900-2200.
Can I find the weekday for a date before 1900 or after 2200?
The calculator intentionally restricts input to 1900-2200 to avoid guessing at historical calendar reforms (many countries switched from Julian to Gregorian on different dates) and to avoid numbers no practical user would plan around. Outside that range, the tool returns an error instead of fabricating an answer.
What is the difference between the weekday index and the name?
The weekday index is a number from 0 to 6 with Monday as 0 and Sunday as 6 — the convention used by most programming date libraries. The name is the human-readable Monday/Tuesday/.../Sunday label. The weekend flag simply tells you whether the index is 5 or 6.
Does the calculator handle leap years correctly?
Yes. It applies the full Gregorian leap-year rule: divisible by 4 is leap, but century years (like 1900, 2100) are not leap unless also divisible by 400 (like 2000). That is why February 29, 2000 was valid while February 29, 1900 and February 29, 2100 are not.
Is this calculator the same as a date difference or week-number tool?
No. A date-difference tool counts days between two dates, and a week-number tool reports ISO week 1-52. This tool only answers one question: given one date, what weekday was it? Use the matching dedicated tool if you need elapsed days or ISO week numbers.
Disclaimer
This day of week calculator is provided for informational purposes only. Weekday results are computed from the proleptic Gregorian calendar and are intended for planning, curiosity, and general reference. They do not account for local holiday schedules, historical calendar reforms in specific countries, or any legal, financial, or contractual obligation. Always confirm critical dates (rent, tax filings, court deadlines, travel bookings) against your own official calendar and relevant authorities.